A disturbance/error is an unknown and uncontrolled input acting on a system.
A fault is an unpermitted deviation of at least one characteristic property or parameter of the system from
the acceptable/usual/standard operating conditions.
A failure is a permanent
interruption of a system’s ability to perform a required function
under specified operating conditions.
Traditional control systems
are designed to return the system to normal operations in the
presence of disturbances but not in the presence of faults or failures.
Fault-tolerant control (FTC) systems refer to control systems
that have been designed to explicitly account for some class of
specified faults in the closed-loop system. FTC systems must act
in the time between a fault and a system failure.
